1. Field of the Invention
This invention relates to an antenna system, and more particularly to a slimmer or shorter antenna system for receiving microwave band signals, such as satellite broadcasting signals.
2. Description of the Related Art
FIG. 1 shows the construction of a conventional antenna system as known, for example, from Japanese Patent Laid-Open Publication No. Sho61-157105 (JPA Sho61-157105). The antenna system is for use in receiving satellite broadcasting signals and the like, and comprised of a main reflector 12 and a primary radiator 14 which are connected by a supporting arm 26. The main reflector 12 is formed by a belt-type parabolic cylinder which is parabolic horizontally and straight vertically, and has a straight focal line in the vertical direction. The primary radiator 14 formed by a micro-strip line is arranged on the focal line of the parabolic cylinder of the main reflector 12. As shown in FIGS. 2 and 3, according to this antenna system, radio waves transmitted from an artificial satellite are reflected by the main reflector 12, and the reflected radio waves are received by the primary radiator 14 arranged on the focal line, whereby the received electric signals are processed by a high frequency circuit 24 directly connected to the primary radiator 14. The antenna system is set, as shown in FIGS. 1 and 3, such that the parabolic cylinder of the main reflector 12 stands vertically in order to prevent snow accretion when installed outside.
When an antenna system as outlined above is applied to a satellite communication apparatus for moving vehicles, it is desirable that the antenna system assume a low profile in order to preserve appearance, reduce crime risk, and reduce wind resistance when traveling. In this regard, the antenna system shown in FIG. 1, is inconvenient in that the larger the elevation angle of the artificial satellite, the more difficult it is to slim the antenna system. More specifically, the antenna system is constructed such that the directivity thereof in the plane at an elevation angle is directed to the artificial satellite by relatively changing the difference between the height of the main reflector 12 and the height of the primary radiator 14. Therefore, the supporting arm 26 is extended as the elevation angle of the artificial satellite is larger, which inevitably increases the entire height of the antenna system (the total height of the height of the main reflector 12 plus the height of the supporting arm 26).